To expect to be consulted on planning major family events...?

[siriusly]

If a major event was happening in your family, hatches, matches, dispatches, etc would you expect the organiser to ask you first if day was ok before sending out invites? i would - just did a big family day out for 20 for Aunt's 80th, and asked all parties beforehand to send an email with dates that were non-negotiable so that I could plan event. Everyone co-operated - day went well. Now, my sibling, has planned a communion and not asked anyone beforehand and is pissed off that we, and others, can't attend. AIBU or AIHighly and perhaps too Overorganised??

siriusly are you sure it's your sister that's a bit of a diva?

No, I wouldn't double check everyone's availability befire an event. I'd book it, send out the invites, and wait and see who can make it.

You sound a bit anal more organised than me

If they didn't check with you they can't complain if you don't come. On the other hand, if this happened to me, I would wherever possible postpone whatever else I was doing and go (I wouldn't cancel a holiday or pull out of a wedding, but most other things could be postponed. I would mutter to DH about it but not other family). I also agree that they probably didn't get much choice as regards the date with the church, and then had to accommodate godparents and grandparents as priority over other relatives and friends. It is very different to organising a family outing which does not have to fit in with an organisation such as the church.

With things like baptisms and communions, the (our) church sets the date, you might get to express a preference for one or two dates, or a time on a set date but that's it.

You and your sister sound close though!!

You sound like me! Highly organised and efficient ! Actually I think that it makes sense to check whether people can attend an event that matters to you before setting a date. If you don't and they can't then you have to live with it. If you've already made arrangements for the date of the christening then it's not your fault it's hers for not checking first.